Experiments with adenovirus 5 indicated that the failure of malignant cells to discriminate against 5-bromodeoxyuridine as a DNA precursor has long-term biological significance. Several steps of DNA repair were examined through the effects of inhibitors of DNA polymerases and topoisomerases on the sedimentation of DNA nucleoids. DNA repair is blocked at its early stages (damage recognition and/or DNA relaxation) by inhibitors of type II DNA topoisomerases, and at its later steps (leading to regeneration of compact, supercoiled DNA) by inhibotors of both alpha and beta DNA polymerases. Both topoisomerase inhibitors produce a 25-35% relaxation of nuclear DNA. Chinese hamster ovary cells that are partially resistant to novobiocin contain DNA nucleoids that sediment 20-30% more slowly that those from the novobiocin-sensitive cells. Their sedimentation is not affected by incubating the cells with novobiocin. The defect in tumor cells that cannot repair alkylation damage (Mer- cells) was found to be multifaceted in studies measuring the relaxation and recoiling of nucleoid DNA in conjunction with collaborative studies measuring induced and constitutive capacity to repair damaged viruses, repair DNA synthesis, and cytotoxicity.